The use of thermoplastic materials for the manufacture of numerous products, including containers, e.g., bags of all varieties, is well known in the art. The use of thermoplastic articles is so widespread that the disposal of thermoplastic articles has become a significant world wide problem. Although it is preferable to recycle as great a portion of these thermoplastic articles as possible into new products, it is acknowledged that at least some finite portion of these articles will ultimately be discarded as unsuitable for recycle applications. The two most traditional mechanisms by which such articles are disposed are by incineration or by depositing the thermoplastic articles in a landfill. The growing use of thermoplastic articles and, accordingly, their ever increasing disposal has resulted in considerable activity relating to methods for controlling the evergrowing volume of thermoplastic materials to be disposed.
Unfortunately, as above noted, recycle programs are not effective for all thermoplastic articles such as the case where the article acts as the container for refuse, such as is the case for household trash, and other types of disposable bags made from thermoplastic films. The particular problems associated with disposer bags is multifaceted owing to the wide array of uses for such bags and, further, owing to the need to manufacture such bags from a multifunctional thermoplastic material that meets both the performance and varied use requirements of today's consumer. In addition, such thermoplastic bags must be disposable in an environmentally responsible way when disposed after the article's useful life.
Although many learned individuals have considered the above disposal problems, it is painfully clear that very few ideas have seen the "commercial light of day". Currently, the disposal problem associated with thermoplastic bags, although only a minute segment of the total quantity of thermoplastic articles disposed of on an annual basis, is being addressed by two different approaches. The first approach is to add a prodegradant, e.g., iron stearate, to the thermoplastic material whereby the rate of photodegradation and/or thermal degradation is increased over that of the original thermoplastic material. Although this approach is extremely useful when the thermoplastic article is placed in sunlight or a thermally activating environment, it is too often the case that the thermoplastic article will be placed in a landfill prior to having significant degradation occur. The second approach involves the use of small amounts of biodegradable fillers for the thermoplastic material. The use of a biodegradable filler typically incorporates a prodegradant also, as above discussed. The biodegradable filler is generally a starch, modified starch or cellulosic component which would be degradable by microorganisms in an oxygen-rich landfill environment. Unfortunately, long term management of landfills has made this approach only a partial solution. It has been observed that many landfills are maintained as dry, anerobic environments as a result of the desire to minimize the leaching of landfill contaminants into ground water. Accordingly, the long term degradation mechanism for the starch component may be halted before it can progress to any meaningful degree. Further, the addition of starch components in amounts up to about ten (10) weight percent have been reported to significantly decrease the strength of thermoplastic film products such as the various polyethylenes. In fact, the net result of adding the starch component will in all likelihood be an overall increase in the amount of thermoplastic employed in manufacturing the article, since an overall increase in the amount of the thermoplastic will be required to maintain the necessary performance characteristics of the thermoplastic article. Further, the addition of a starch or other component as a "filler" for a thermoplastic article does not render the thermoplastic material biodegradable. The thermoplastic material remains after the biodegradable filler is removed, although the form of thermoplastic article may fragment to some extent as a result of the degradation of the biodegradable component.
The aforementioned considerations have resulted in numerous patents and voluminous prior art relating to degradable thermoplastic materials. A review of the more interesting and relevant prior art is considered hereinafter.
U.S. Pat. No. 4,016,117 is directed towards a biodegradable synthetic resin composition which includes a polymer, a biodegradable filler (which may be starch, as set forth in claim 2), and an auto-oxidizable substance, which may be a fatty acid, which auto-oxidizes when in contact with a transition metal salt. It is said that the composition degrades in the soil, although clearly it is only the biodegradable filler that undergoes biodegradation.
U.S. Pat. No. 4,256,851 is directed towards an environmentally degradable plastic composition which comprises an admixture of a polyolefin and an ethylenically unsaturated alcohol. Claim 6 recites a specific antioxidant.
U.S. Pat. No. 4,021,388 is a divisional of U.S. Pat. No. 4,016,117, discussed above. The '388 patent has claims directed towards a biodegradable composition which includes a polymer having carbon to carbon linkages with starch granule dispersed therein. The claims further recite that the surface of the granules are modified by reaction with a compound which reacts with hydroxyl groups to form an ether or an ester.
U.S. Pat. No. 4,133,784 has claims directed towards a biodegradable film composition which comprises a dry composite of a starchy material and a water-dispersable ethylene acrylic acid copolymer.
U.S. Pat. No. 4,337,181 has claims which are directed towards a biodegradable film and method of making such a film. The method involves mixing a gelatinized starchy material with an ethylene acrylic acid copolymer to produce a plasticized matrix which is then shaped into a film. The method further includes the steps of adding an ammonia neutralizing agent to the mixture, adjusting the moisture content to a specified range and extrusion blowing the mixture into a film.
U.S. Pat. No. 4,454,268 is directed towards a semi-permeable film and method of making such a film. The film includes an ethylene acrylic acid copolymer and a starchy material.
U.S. Pat. No. 4,324,709 has claims directed towards a plastic-based composition and method of making such a composition. The composition includes a synthetic polymer and starch granules which have a modifying substance absorbed therein.
U.S. Pat. No. 3,935,141 discloses an environmentally degradable ethylene polymer composition comprising: about 0.01 to about 40 percent by weight of at least one auto-oxidative susceptible additive; about 0.002 to about 2.0 percent by weight of the metal atom of at least one salt of at least one polvalent metal selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc and cerium; and about 0.0025 to about 1 percent by weight of an antioxidant for the ethylene polymer. The patentee discloses the use of organic salts of the polyvalent metals, including stearate, octoate, dodecyl acetoacetate, oleate and distearate. In selecting the antioxidant the patentee discloses that commonly employed antioxidants may be employed, as described at column 8, line 19 to column 9, line 12. The patentee notes, at column 2, lines 40 to 51, that one can proportion the amounts of antioxidant and additives to give a structurally stable product during the useful life period of the product but which will undergo embrittlement within a relatively short time after exposure to the elements.
PCT/GB88/00386 (PCT application), to Griffin discloses a degradable polymer composition wherein an antioxidant and pro-oxidant are combined whereby as the antioxidant is stated to be depleted with time. After total depletion of the antioxidant a residual concentration of prooxidant remains and is stated to provide a sharp loss in the physical strength of the degradable polymer composition. The application discloses use of a stable polymer such as low-density polyethylene, linear low density polyethylene, high density polyethylene, polypropylene, or polystyrene, together with a less stable unsaturated elastomer type compound made from the polymerization of 1:4 dienes, or the copolymerization of such 1:4 dienes with ethenoid comonomers, such as styrene-butadiene elastomer or natural rubber. The invention embodies less stable substances capable of auto-oxidation because such substances are more readily initiated by the oxidation process by virtue of their unsaturation. Once the auto-oxidation process is initiated, the process is said to involve more resistant saturated substances such as polyolefins, although no data is disclosed to support this claim.
Although not directly related to degradable thermoplastic compositions, several patents are of interest owing to their disclosure of multilayer films. U.S. Pat. Nos. 4,254,169 and 4,239,826 disclose multilayer barrier films having a core layer of ethylene vinyl alcohol polymers or vinyl alcohol polymers or copolymers thereof. The patentee in U.S. Pat. No. 4,254,169 discloses that a delamination resistant multilayer structure of a barrier layer comprising a substantially pure polymer or copolymer of vinyl alcohol may be formed with an adhered layer of a modified polyolefin which consists essentially of a polyolefin polymer or copolymer blended with a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride. Modified polyolefin blends of this type are discussed by the patentee in column 3, lines 1 to 21 wherein the patentee discloses that such modified polyolefin blends are available under the name PLEXAR.RTM.. The patentee discusses the nature of the multilayer barrier film and the goals to be achieved in several ways. For example, as noted at column 2, lines 42 to 53, because of the core layer's excellent barrier properties, the core layer can be made quite thin, in the range of 0.1 mil or less. The patentee views this as an advantage which results from using the modified polyolefin blends as an outer film layer. Further, at column 2, lines 20 to 38, the patentee characterizes the multilayer barrier film for its high resistance to the passage of gas and moisture. The patentee further states that the invention is preferrably manufactured by cast coextrusion, but can be manufactured by blown film coextrusion, water quench coextrusion, or lamination and that the resultant film can be satisfactorily heat sealed.
U.S. Pat. No. 4,640,852 is directed towards a multiple layer film (see: Claim 2) which comprises a nylon layer, an ethylene vinyl alcohol copolymer layer, and a layer of nylon.
Another area of interest relating to the use of degradable thermoplastic bags is in the field of composting. Although the prior art has broadly addressed the issue of degradable polymers, the issue of use of degradable thermoplastic bags in composting has not received wide spread attention. In the majority of current composting applications a Kraft paper leaf bag has been employed. Typically, if a thermoplastic bag has been employed to contain yard waste a preparatory step of debagging the yard waste from the thermoplastic bag has been required. In an attempt to improve the utility of thermoplastic bags in composting the use of biodegradable additives, e.g., starch, in the thermoplastics has been suggested. Unfortunately, as above discussed, the addition of small amounts, typically on the order of 1 to 10 weight percent, of a biodegradable additive does not make the remaining thermoplastic material degradable, although it may result in some loss in the tensile strength of the thermoplastic bag. A thermoplastic bag specifically designed for use in composting is needed.
The instant invention is directed towards the problem of improving the degradation of thermoplastic articles when such are subjected to environmental conditions as encountered in landfills and composting.